Translation:Which is the smallest dress that you sell?
"Mana" = "where". "Which" = "yang mana". Therefore, the English translation in this case should be "Where is the smallest dress that you sell?"
Yes, that is true literally, but I don't think I'd hear someone ask this way in English. "where" is more common, in this context, which, I think, doesn't help the confusion here. If the sentence was "Di antara dua gaun ini, mana yang terkecil anda jual?" it might be easier.